1. Field of the Invention
This invention relates to a circuit for monitoring battery voltage in an implanted cardiac pacemaker, and for indicating when a preselected battery depletion level is exceeded.
2. Description of the Prior Art
Typically, the useful service life of an implantable cardiac pacemaker is limited by the depletion characteristics of the battery power source which powers the pacemaker. As the battery depletes the output voltage decreases and the internal impedance of the battery increases. The increased impedance limits the amount of current which can be drawn from the battery by the pacemaker. Eventually the pacemaker will cease functioning because of inadequate battery voltage or insufficient current. It is desirable to provide a warning indication to notify the patient and his physician that the end of the useful service life of the pacemaker is being approached so that the failing pacemaker may be replaced before it ceases functioning.
In many prior art pacemakers the asynchronous pulse rate progressively decreases as the battery depletes. This reduction in pulse rate is used as the depletion indicator. Elective replacement of a pacemaker would typically take place when the measured asynchronous pulse rate has decreased by ten percent. However, recently introduced digital pacemakers, which are crystal controlled, do not undergo a progressive change in pulse rate as the battery depletes. As a consequence, additional circuitry must be added to these pacemakers to initiate an end-of-life indicator.